The invention relates to a piston crown for a piston in an internal combustion engine arrangement comprising a cylinder, the piston crown having a piston bowl surface adapted for facing a combustion chamber in said cylinder, wherein the piston bowl surface comprises:
a circumferential rim portion, a floor portion connected to and surrounded by said circumferential rim portion, a plurality of circumferentially spaced protrusions in the circumferential rim portion, and at least one spray impingement portion, located between two adjacent protrusions.
The invention can be applied in internal combustion engine arrangements, in particular in a diesel engine, for all types of vehicles. As such, the invention may be applied in an internal combustion engine arrangement for a heavy vehicle such as a lorry or truck, a construction equipment vehicle, a marine vessel or a car.
In the field of internal combustion engine arrangements, numerous efforts are made to accomplish efficient combustion which is also satisfactory in view of residual products, in particular soot particle and NOx emissions, although also carbon monoxide emissions, and hydrocarbon emissions may naturally be considered.
A combustion process in which the fuel is injected directly into the cylinder and is ignited by increased temperature and pressure in the cylinder is generally referred to as a diesel process. When the fuel is ignited in the cylinder, combustion gases present in the cylinder undergo turbulent mixing with the burning fuel, so that a mixture-controlled diffusion flame is formed. The combustion of the fuel/gas mixture in the cylinder gives rise to heat generation which causes the gas in the cylinder to expand and which hence causes the piston to move in the cylinder. Depending on a number of parameters, such as the injection pressure of the fuel, the quantity of exhaust gases recirculated to the cylinder, the time of injection of the fuel and the turbulence prevailing in the cylinder, different engine efficiency and emission values are obtained.
The production of soot particles during combustion is a complicated process, where the quantity of soot particles is the net difference between formed soot and oxidised soot. Generally, combustion performed with a fuel-rich fuel/air mixture, insufficient mixing and at high temperature produces high soot formation. However, the formation of soot particles may be counteracted if soot oxidation may be promoted.
During combustion, nitrogen oxides (NOx) are formed from the nitrogen content in the air in a thermal process having a strong temperature dependency, and which inter alia depends on the size of the heated-up volume and the duration of the process.
Also, carbon monoxide emissions (CO) and hydrocarbon emissions (HC) may appear, in particular if unburnt fuel ends up in relatively cool regions of the combustion chamber, such as close to the cylinder wall or in cavities between piston and cylinder lining.
To control and in particular to reduce emissions from the combustion process in a combustion engine, it has been proposed to utilise the shape of the piston bowl surface facing towards the combustion chamber. The piston bowl surface is part of a piston crown of the reciprocating piston in a cylinder. To this end, the piston bowl surface may be designed so as to affect various parameters inside the combustion chamber such as flame propagation, mixing energy, kinetic energy distribution, and/or swirl. Generally, it has been desired to minimise the occurrence of stagnation zones in the combustion chamber.
For certain applications, it has been proposed to use piston crowns comprising circumferentially arranged protrusions or ridges for influencing the conditions inside the combustion chamber.
For example, WO 2011/101154 describes a piston arranged for reciprocal movement in a combustion engine cylinder, wherein, half way between flame plume impingement areas and in a plane perpendicular to said reciprocal movement, there are arranged protrusions or ridges protruding into the combustion chamber.
U.S. Pat. No. 8,499,735 describes another piston, wherein, between spray/flame plume impingement areas and in a plane substantially perpendicular to reciprocal piston movement, there are arranged a first type of protrusions protruding into the combustion chamber, having a smooth form for preserving kinetic energy in the flame and for redirecting circumferential flame progress mainly towards a center axis of the piston with minimal flame-to-flame interaction. Moreover, protrusions of a second type are arranged in the impingement areas, which are adapted for redirecting the flame towards a circumferential flame progress direction.
There is a need to further improve the control of the combustion process in a combustion engine arrangement, or to provide a useful alternative.
It is desirable to provide a piston crown fulfilling the above-mentioned need.
Hence, there is provided a piston crown for a piston in an internal combustion engine arrangement, the piston crown having a piston bowl surface adapted for facing a combustion chamber in said cylinder, wherein the piston bowl surface comprises: a circumferential rim portion, a floor portion connected to and surrounded by said circumferential rim portion and a plurality of circumferentially spaced protrusions in the circumferential rim portion, and at least one spray impingement portion, located between two adjacent protrusions.
The spray impingement portion comprises a reflection surface, being defined by that each possible normal to the reflection surface is directed towards a central axis (A) of the piston, and forming an angle being in the range of a constant angle ±10° with said central axis.
Further, the constant angle is at least 50°.
As implied by the above-mentioned definition, the reflection surface is defined as a portion of the rim surface displaying a variation of ±10° about a constant angle. Also, it is implied in the term “surface” that the reflection surface should have at least some extension along the central axis and circumferentially about the central axis.
The reflection surface in the spray impingement portion is the surface onto which a spray originating from an orifice in an injector is intended to impinge, causing the spray to be reflected. By affecting the way in which the spray is reflected, the shape of the reflection surface will impact the distribution of the kinetic energy inside the combustion chamber.
The provision of a reflection surface as defined in the above in the spray impingement portion may be made in an arrangement of the piston crown together with an injector for injecting a spray along at least one spray vector. Hence, the spray vector will form an angle with the central axis. The reflection surface should be designed in correspondence with the desired spray vector, such that the constant angle of the reflection surface corresponds to the angle which the spray vector forms with the central axis.
In this case, the reflection surface will be generally perpendicular to the spray vector, resulting in the impinging spray being reflected back towards the opposite direction of the spray vector. A reflection angle being defined between the spray vector impinging upon the reflection surface, and a general reflected spray direction, will hence be approximately zero.
This is believed to be particularly advantageous in combination with the spray impingement area being neighbored by protrusions as described in the above.
Indeed, with such a piston crown in an internal combustion engine arrangement, results have been achieved indicating advantageous fuel consumption properties as seen in relation to the compression volume of the internal combustion engine arrangement. Moreover, such an arrangement has been shown to be advantageous in view of soot emissions.
The constant angle being at least 50° implies that the spray vector may form an angle of at least 50° with the central axis.
Accordingly, use of a relatively large angle between the central axis and the injecting spray vector is envisaged. This is believed to be particularly advantageous in combination with the protrusions and the spray impingement areas as described herein, allowing for favorable combustion conditions.
Optionally, the reflection surface substantially has the shape of a portion of an envelope surface of an imaginary cone, preferably a right circular cone. The above-mentioned shape of the reflection surface implies that the impinging spray may be reflected in a desired direction.
Optionally, the imaginary cone has a cone apex and a cone base. The cone apex and the cone base are located on opposite sides of the floor portion, Wherein the floor portion faces the cone base.
Optionally, the constant angle may be at least 60°, preferably at least 70°, most preferred between 70 and 80°.
Optionally, the constant angle may be less than 85°, preferably less than 80°.
According to embodiments, the reflection surface may be defined by that each possible normal to the reflection surface is directed towards a central axis of the piston, and forming an angle being in a range of a constant angle ±5° with said central axis, preferably forming an angle being in the range of a constant angle ±2° with said central axis.
If such a piston crown is arranged with an injector providing a spray vector forming said constant angle with the central axis, this means that a reflection angle being defined between the spray vector impinging upon the reflection surface, and a general reflected spray direction will be limited to 5°, or 2°, respectively (considering the entire reflection surface).
In particular, the reflection surface may be defined by that each possible normal to the reflection surface is forming an angle being the constant angle. In this case, in combination with a spray vector likewise forming the constant angle with the central axis, a reflection angle may be approximately 0°, all over the reflection surface.
In a second aspect, an internal combustion engine arrangement is provided comprising an engine cylinder, a piston crown having a piston bowl surface facing a combustion chamber in said cylinder, the piston bowl surface comprising:
a circumferential rim portion, a floor portion connected to and surrounded by said circumferential rim portion, a plurality of circumferentially spaced protrusions in the rim portion, at least one spray impingement portion, located between two adjacent protrusions, and an injector comprising at least one orifice, said orifice being arranged to inject fuel along a fuel spray vector towards said spray impingement portion.
In the internal combustion engine arrangement, the spray impingement portion comprises a reflection surface, the reflection surface being defined by that each possible normal to the reflection surface forms an angle γ with the spray vector of the corresponding injector orifice, as seen in a plane through the central axis, the angle being less than 10°.
Accordingly, a theoretical reflection angle being the difference between the spray vector injected towards the reflection surface, and a spray leaving the reflection surface after having impinged towards it, should be less than 10°.
Advantageously, the angle between each possible normal to the reflection surface and the spray vector of the corresponding injector orifice, as seen in a plane through the central axis is less than 5°, preferably less than 2°.
In particular, the angle between each possible normal to the reflection surface and the spray vector of the corresponding injector orifice, as seen in a plane through the central axis may be essentially zero. In this case, the reflection surface will be formed as a part of a conical surface, for which all possible normals are parallel to the spray vector.
As an alternative, the fuel spray vector forms a spray angle the with the central axis and the constant angle is at least 2°, preferably at least 5°, smaller than the spray angle. This in turn implies that fuel will be reflected towards a position above the injector orifice, as a consequence of which a combustion flame during combustion will form a somewhat sharp angle with the central axis and this may improve the efficiency of the internal combustion engine arrangement.
For both aspects of the invention, whether considering the piston crown only or considering an internal combustion engine arrangement, the following features may be provided.
Generally, the rim portion defines a maximum level of said piston bowl surface along the central axis, and the floor portion defines a minimum level of said piston bowl along the central axis, the piston bowl having a piston bowl extension along said central axis being the difference between said maximum level and said minimum level, and wherein said reflection surface has a reflection surface extension along the central axis.
According to an embodiment, the reflection surface extension is at least 10% of said piston bowl extension, preferably at least 15% of said piston bowl extension.
According to an embodiment, the reflection surface extension is between 15 and 30% of said piston bowl extension, preferably between 20 and 30%, most preferred about 25%.
The axial extension of the reflection surface (the reflection surface extension) should preferably be such that the spray vector will impinge upon the reflection surface during at least a main portion of the injection period. Preferably, the spray vector will impinge upon the reflection surface during the entire injection period. Accordingly, a suitable axial extension of the reflection surface may be selected by considering the injection period and the distance along the central axis which the piston crown will move in relation to an injector orifice during said injection period.
According to an embodiment, the reflection surface extension is extending at a first distance from the minimum level of said piston bowl along the central axis, said first distance being at least 15% of said piston bowl extension, preferably at least 25% of said piston bowl extension, most preferred at least 30%. Advantageously, the first distance may be less than 50% of said piston bowl extension, preferably less than 40%.
The reflection surface will furthermore have an extension along the circumferential direction of the rim portion. The extension along the circumferential direction may be expressed in terms of an angle about the central axis, in a plane perpendicular to said central axis.
The circumferential extension of the reflection surface should generally be selected such that the area provided by the reflection surface is sufficient for the entire spray jet to impinge thereupon.
In an embodiment, the reflection surface may have a circumferential extension of at least 5° about said central axis, in a plane perpendicular to said central axis. Preferably, the circumferential extension is at least 10°, most preferred in the range 10 to 40°. Optionally, the circumferential extension is less than 50°, preferably less than 40°.
In an embodiment, the rim portion may comprise at least two reflection surfaces, more preferred 2 to 10 reflection surfaces, more preferred 4 to 8, most preferred 5 to 7. The amount of reflection surfaces may naturally be adapted to the number of spray jets, i.e. the number of injector orifices of a corresponding injector.
In an embodiment, the reflection surfaces may be located at regular angular intervals about the central axis. Also the location of the reflection surface may naturally be adapted so as to correspond to the location of injector orifices of a corresponding injector.
In an embodiment, all reflection surfaces of the piston crown are uniform.
In an embodiment, at least some of the protrusions are uniform, preferably all of said protrusions of the piston crown are uniform.
In an embodiment, at least the portion that is located closest to the central axis of at least one of said protrusions forms a surface, for which each possible normal forms the same angle as the reflection surface.
Advantageously, the inner shape of the piston bowl may be designed such that the reflection surfaces join smoothly with the protrusions. For example, the reflection surfaces may be seen as portions on a circumferentially extending band, having the axial extension Da and being arranged so as to form a part of the inner contour of the piston bowl, extending around the circumference thereof. Such a band would hence be applied so as to form the contour of the protrusions as well as the reflection surfaces in between the protrusions.
The protrusions will extend towards the central axis A at least a distance Ddiff from the reflection surfaces. For example, the distance Ddiff may be at least 10% of the maximum inner radius of the piston bowl surface, advantageously at least 20%. Preferably, the distance Ddiff may be less than 50% of the maximum inner radius of the piston bowl surface.
Also, the protrusions will have an extension along the circumference of the piston bowl. The extension may be at least 5°, preferably at least 10°, most preferred between 10 and 30°.
In an embodiment, an inner circumferential wall of said piston bowl may consist of or comprise said protrusions and reflection surfaces.
Optionally, the piston bowl surface may comprise a concave surface, positioned between the floor portion and the reflection surface, and a convex surface, positioned between the reflection surface and the circumferential rim portion.
In another aspect, the present disclosure relates to a piston comprising a piston crown in accordance with the above.
In another aspect, the present disclosure relates to an internal combustion engine arrangement comprising an engine cylinder and a piston positioned for reciprocal movement in the engine cylinder along a central axis (A) between a bottom dead center position and a top dead center position, wherein the piston comprises a piston crown as described in the above.
Preferably, the piston crown is arranged in relation to an injector of said combustion engine arrangement, said injector comprising at least one orifice, said orifice being arranged to inject fuel along a fuel spray vector towards said spray impingement portion such that each possible normal to the reflection surface forms an angle with the spray vector of the corresponding injector orifice, as seen in a plane through the central axis, the angle being less than 10°, preferably less than 5°, preferably less than 2°, most preferred substantially 0°.
As already described in the above, an internal combustion engine arrangement wherein the angle between the normal to the reflection surface and the spray vector of the corresponding injector orifice is less than 10°, preferably less than 5°, preferably less than 2°, most preferred substantially 0° may optionally possibly be achieved using a piston crown where the constant angle is more than or equal to 50°.
In another aspect, there is provided a vehicle, preferably a truck, comprising an internal combustion engine arrangement comprising a piston crown as described in the above and/or an internal combustion engine arrangement as described in the above. It will be understood that the present disclosure may also be applied to other engines than a vehicle engine in a truck, e.g. to an engine in a bus, excavator, loader, or other vehicle, or to an engine for marine applications, for providing back-up power etc.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.